Direct-view storage tube and method of erasure



K. R. HESSE DIRECT-VIEW STORAGE TUBE AND METHOD OF ERASURE Filed April2v, 1964 atet 3,331,983 Patented July 18, 1967 3,331,983 DIRECT-VIEWSTORAGE TUBE AND METHGD F ERASURE Kenneth R. Hesse, Oceanside, Calif.,assiguor to Hughes Aircraft Company, Culver City, Calif., a corporationof Delaware Fiied Apr. 27, 1964, Ser. No. 362,626 Claims. (Cl. 315-12)ABSTRACT 0F THE DISCLOSURE Direct-viewing storage tube employing astorage target having a mesh electrode and a storage dielectric thereonfacing the scanning and flood electron guns. The storage dielectricexhibits both lield induced and controlled conductivity and, by means ofan electric field established across the dielectric, the leakage ofelectrical charges to the mesh electrode and hence erasure of thedisplay may be selectively controlled.

This invention relates to electronic storage devices and especially todirect-viewing storage tubes capable of halftone presentations of giveninformation. More particularly, the invention relates to methods andmeans for erasing storage displays in such a tube and especially forestablishing the rate of erasure and/or the duration of the display.

One of the types of direct-view storage tubes to which the presentinvention relates is described in U.S. Patent 2,790,929 to E. E. Hermanand G. F. Smith, assigned to the instant assignee. This storage tubecomprises a target assembly which includes a storage target and aviewing screen. Two electron guns are provided: one gun (hereinaftercalled the writing gun) causes the formation of a charge pattern on thestorage target corresponding to the information to be presented; theother gun (hereinafter called the flood or viewing gun) renders thecharge pattern visible on the viewing screen. Information-representativesignals are employed to modulate the intensity of the electron beamproduced by the writing gun. The storage target is initially uniformlycharged negatively. When the writing beam strikes the storage target, aless negative potential is produced at the point of impingement by thephenomenon of secondary emission. It should be understood that as usedherein in describing the storage or charge pattern on the storagetarget, the term positive designates those portions which are lessnegatively charged than other portions of the target. The potentials onthe storage target are always negative with respect to the potential ofthe flood gun cathode except when erasing. Thus by scanning the storagetarget an overall'charge pattern corresponding to the information to bepresented is obtained. Thereafter this charge pattern controls thepassage of flood electrons from the viewing gun to the viewing screen;ood electrons, properly collimated over the entire area of the storagetarget, are only passed through areas of the storage target which yareless negatively charged. It should be understood that the potentialscapable of being stored range from a uniformly negative black value to auniformly positive white value: at intermediate potentials only aportion of the flood electrons passes through the storage element to theviewing screen thereby producing intermediate or half-tone shades.

The present invention may also be used in direct-viewing half-tonestorage tubes of the type described in U.S. Patent 3,086,139 to N. H.Lehrer, assigned to the instant assignee, often referred to as amulti-mode storage tube because of the ability of this tube to presentboth stored and non-stored displays and to selectively erase portions ofthe stored display.. In such a multi-mode storage tube, the phenomenonof bombardment induced conductivity as well as secondary emission isutilized to provide the storage mechanism. The bombardment inducedconductivity storage target is responsive to the energy level of anelectron beam impinging thereon whereby at one beam energy level theprincipal effect is secondary electron emission greater than unity so ast-o permit the development of an electrostatic storage patterncorresponding to that utilized in the aforementioned patent to E. E.Herman and G. F. Smith. At a dilferent energy level the principal eifectis bombardment induced conductivity which permits the storage target tobe charged in an opposite electrical sense so that previously storedcharges may be selectively erased by scanning the target with a beam atthis second energy level. It is also possible, by utilizing an electronbeam having an intermediate energy level, to cause some of the beam topass through the storage target without having the portion of the beamwhich strikes the storage target appreciably alter the stored chargepattern or yother potential conditions of the storage dieelectric. Thus,the electron beam of such intermediate energy level can be used toestablish the display of nonstored or live information simultaneouslywith the stored information. In both of the types of tubes described,display Iof the stored information is achieved by the penetration of oodelectrons through the storage target in accordance with the storagepotentials thereon.

In both of the aforementioned types of storage display tubes, erasure ofthe stored information may be achieved by the application of a train ofpulses to the backing electrode for overall or general erasure. In themultimode storage tube described above, selective erasure is alsopossible by utilizing a high energy writing gun as mentioned. In eitherinstance, both tubes require the utilization of additional circuitry toobtain general or overall erasure v which thereby adds complexity to thedisplay system.

It is therefore an object of the present invention to provide adirect-viewing storage tube the duration of wh-ose stored displays maybe -controllably established with a minimum amount of complex circuitry.

Another object of the present invention is to provide an improveddirect-viewing storage tube capable of having the stored displaysthereon erased by means other than electrical pulses applied to thebacking electrode thereof.

Yet another object of the present invention is to provide adirect-viewing storage tube having improved erasing means.

These and other objects and advantages of the invention are realized byproviding a direct-viewing storage tube with a storage target whichexhibits field-induced conductivity and/or sustained bombardment inducedconductivity. By maintaining the backing electrode of the storage targetsuticiently negative, field-induced conductivity currents are caused toow through the storage insulator. Positive areas are formed or writtenby means of secondary emission and a scanning electron or writing gun,for example. These positive areas will be gradually discharged towardthe backing electrode potential at a rate dependent upon the eld acrossthe insulator material. Hence, by varying the voltage on thebacking-electrode, this discharge rate may be controllably establishedso that stored displays having persistencies in the range 0f fractionsof seconds to many seconds may be achieved.

Another advantage attendant with the use of the present invention is theimproved contrast of the displays which result from the method oferasure. The method of erasure used in the direct-view storage tubes ofthe prior art ernploying the ood electrons to achieve erasure resultedin a general or overall background light due to the erase pulses appliedto the backing electrode. Since the background brightness and associatedcontrast ratio are dependent upon the duty cycle of the erase pulsetrain, the

higher the duty cycle, the higher the background brightness and thepoorer the contrast ratio between stored information and the background.Such degradation is avoided, in the prior art, by applying a pulse tothe viewing screen in synchronism with the erase pulse. However, theamplitude of such pulse must be of the order of 6,000 to 8,000 voltswhich produces an increase in both the complexity of the necessarycircuit and in radio frequency interference radiation. In employing thepresent invention, contrast is not only improved but these disadvantagesare avoided since flood electrons are not used for erasure.

The invention will be described in greater detail by reference to thedrawings in which the sole figure is a schematic diagram of adirect-viewing half-tone storage tube device according to the presentinvention.

Referring now to the drawing, a direct-viewing storage tube is shownwhich comprises an evacuated envelope 20 having an enlarged cylindricalportion 21 and a neck portion 22. The end portion 33 of the cylindricalportion 21 is generally flat and transparent. Disposed adjacent the atend portion 33 is the target assembly according to the present inventioncomprising a viewing screen 24, and a storage screen 26. An electron gun30, hereinafter called the writing gun, is disposed in the neck portion22 of the envelope 20. Also disposed in the neck portion and adjacentthe electron gun 30 is a deflection system 31 for directing the electronbeam produced by the writing gun 30 over selected elemental areas on thestorage screen 26. While an electrostatic deflection system is shown inthe drawing comprising horizontal and vertical deliecting plates 32 and34, respectively, deflection of the beam produced by the electron gun 30could also be achieved electro-magnetically as is Well-known in thecathode ray tube art. The electron beam produced by the writing gun isof elemental cross-sectional area.

More particularly, the writing electron gun 30 includes a cathode 36 anda grid 38 which controls the intensity of the writing electron beam inaccordance with signals supplied thereto by a connection from the signalsource 12. Illustratively, the cathode 36 may be maintained at apotential of about several thousand volts negative with respect toground. An adjustable quiescent potential of about 50 volts negative,for example, with respect to the cathode potential may be maintained onthe intensity grid 38.

By means of appropriate connections, horizontal and vertical deectionpotentials may be applied to the electrostatic deection system 31. Thesedeflection potentials may be developed by a deection generator 14 insynchronism with signals through a connection to the signal source 12.

The viewing screen 24 of the target assembly comprises a phosphorcoating 68 disposed on the inner surface of the flat transparent endportion 33 of the envelope 20. An electron transparent conductivecoating or lm 70 may be then disposed over the phosphor coating 68. Theconductive coating '70 may be a thin film of aluminum, for example. Theconductive coating 70 may be maintained at a potential of from 5 to l0kv. by means of a connection to the positive terminal of a battery 72whose negative terminal is connected to ground.

The storage screen 26 comprises an electroformed nickel mesh 76 having,for example, about 250 linear openings per inch and a thickness of theorder of 0.001". The optical light transmissivity of this screen isapproximately 40% to 50%. The outer periphery of the screen may beWelded t-o a support ring 74. A coating 78 is provided on the surface ofthe screen facing the electron gun 30. This coating is produced from adielectric material which exhibits the properties of both secondaryelectron emission and field-induced conductivity. A typical material iscalcium uoride applied to a thickness of from 0.3 to 0.6 micron, forexample.

The preferred method of application of the calcium uoride coating 78 isby vacuum evaporation and condensation. This method is well known in theart and is described by L. Holland in Vacuum Deposition of Thin Films,published by John Wiley & Sons, New Yorkf Briey, a small amount ofcalcium uoride, approximately one gram, is placed in a molybdenum boat.This boat is formed of thin molybdenum sheet, 0.003 thick, and is madeapproximately 3% wide and 2" long. The ends of the boat are clampedbetween two electrodes by which a low voltage current is passed throughthe boat for the purpose of heating the same. The .storage screen andthe boat are disposed in a vacuum with the screen positioned over theboat. Sufficient spacing between the boat and the screen is provided sothat the condensation of the dielectric material occurs in a uniformlayer upon the storage screen. Approximately 13 inches has been foundadequate for a nominal 5-inch diameter screen.

A vacuum of the order of l06 torr is maintained in the bell jar duringthe evaporation of the calcium fluoride. Total evaporation time isapproximately 20 to 40 minutes for the deposition of a 0.50 micron thicklayer.

The st-orage screen 26 is disposed adjacent the viewing screen 24 and isco-extensive therewith and may be positioned at approximately 0.150 awayfrom the viewing screen, for example. The storage screen 26 may beconnected through a variable resistor 82 to the negative terminal of abattery 84 as shown. Disposed adjacent the storage target 26 is acollector screen or target member 28 which may be maintained at apositive potential of about volts with respect to ground by means of aconnection to a tap 62 of the potentiometer 63.

In operation, upon the application of appropriate signals to the writingelectron gun 30 and its associated deflection system 32, the electronbeam formed by the electron gun 30 is intensity modulated and caused tostrike the storage screen 26 with such energy that the secondaryemission from this surface is very high; i.e., many secondary electronsare emitted for each incident primary electron. These secondaryelectrons are attracted away from the storage surface to the collectorgrid 23, thus causing the formation of a charge pattern on the storagesurface in accordance with the intensity of the scanning electron beamwhich, in turn, is modulated according to the applied intelligencesignals.

Thus, those areas of the storage surface which are charged positive inthis manner with respect to the Hood gun cathode 50 permit the floodelectrons therefrom to pass through the storage screen assembly tostrike the viewing `screen thereby producing a visual presentation, inaccordance with the number of impinging flood electrons which number, inturn, depends upon the potentials -of the storage surface.

In a typical embodiment, a quiescent potential of about 5 volts negativewith respect to ground may be maintained on the storage screen 76. Theinitial quiescent potential established on the storage surface 73 iszero. In this state, the storage screen transmits the maximum number ofood electrons and the tube is at its maximum brightness. (Previously, itwas customary to erase stored `charges from the storage surface and torestore it to its quiescent potential by periodically pulsing thestorage screen 26 with a series of pulses to achieve a slow fade-out orwith a single pulse to achieve instantaneous erasure.)

According to the present invention, however, the potential of thestorage screen or backing electrode 76 is increased or made morenegative (i.e., -10 volts) with respect to the flood gun cathode so thatflood electrons cannot penetrate therethrough except where positiveareas are established on the storage surface by the writing gun. Bymeans of this negative potential on the backing electrode, field inducedcurrents are caused to ow through the storage dielectric 78 so that thepositive areas thereof, written `by means of secondary emission with thewriting gun 30 as described previously and at compartively low energy(i.e., about 2,000 volts), are gradually discharged toward the backingelectrode potential at a rate determined by the field established acrossthe storage dielectric 78. The voltage established on the backingelectrode 76 permits the rate of erasure to be controlled so as toachieve persistencies of display ranging from fractions of a second tomany seconds. The exact voltage which must be applied to the backingelectrode 76 to produce a specific persistence, is primarily dependentupon the thickness of the dielectric layer. Thus, with a dielectriclayer of about 0.3 to 0.6 micron in thickness, and a backing electrodepotential of about volts negative, as described, a persistency of abouttwo seconds is obtained. The backing electrode Voltage range may be fromapproximately 5 volts to 25 volts negative with respect to flood guncathode to obtain persistencies of from about 200-300 milliseconds toabout -20 seconds.

In this manner the leakage rate of the stored positive charges iscontrolled so as to establish displays of any desired period orpersistency which displays fade and are erased automatically accordingto any desired erasure of fade program. In this connection the inventionis related to the erasure technique taught in Iams 2,259,507 except thatIams taught that diiferent storage or erase periods could be obtainedonly by changing the dielectric material itself so as to provide storagedielectrics of dierent specific resistivities to obtain the desired rateof leakage. Thus, the present invention is a considerable improvementover Iams since the leakage rate may be changed at will in a givenstorage tube without having to replace the storage dielectric.

What is claimed is:

1. A direct-viewing half-tone storage display tube coniprising, incombination:

(A) a first electr-on gun for providing a scanning electron beam ofelemental cross-sectional area;

(B) a second electron gun for providing a iiood beam of electrons;

(C) a viewing target positioned within the target area of said electronguns adapted to luminesce in response to the impingement of electronsthereon;

(D) a storage target disposed between said electron guns and saidViewing target and including:

(l) an electron permeable electrode member;

(2) a storage dielectric layer on the surface of said electrode memberfacing said electron guns and exhibiting eld induced and controlledconductivity;

(E) means including said storage target for forming a pattern ofelectrical charges `on the surface of said dielectric layer in responseto the impingement theret on of said scanning electron beam;

(F) means for selectively controlling the leakage of said electricalcharges to said electrode member comprising means for establishing apredetermined electrical field across said dielectric layer.

2. A direct-viewing half-tone storage display tube comprising, incombination:

(A) a rst electron gun for providing a scanning electron beam ofelemental cross-sectional area;

(B) a second electron gun for providing a ood beam of electrons;

(C) a viewing target positioned within the target area of said electronguns adapted to luminesce in response to the impingement of electr-onsthereon;

(D) a storage target disposed between said electron gun and said viewingtarget and including:

`( l) a mesh electrode member;

(2) a secondary emissive storage dielectric layer 4on the surface ofsaid mesh electrode member facing said electron guns and exhibiting eldinduced and controlled conductivity;

(E) means including said storage target for forming a pattern ofelectrical charges on the surface of said dielectric layer in responseto the impingement thereon of said scanning electron beam;

(F) means for selectively controlling the leakage of said electricalcharges to said electrode member comprising means for establishing apredetermined electrical field across said dielectric layer.

3. A direct-viewing half-tone storage display tube comprising, incombination:

(A) a first electron gun for providing a scanning elec- Y tron beam ofelemental cross-sectional area;

(B) a second electron gun for providing a tiood beam of electrons;

(C) a viewing target positioned within the target area of said electronguns adapte-d to luminesce in response to the impingement of electronsthereon;

(D) and a storage target disposed between said electron guns and saidviewing target and including:

( l) a mesh electrode member; (2) a bom-bardment induced conductivitystorage dielectric layer on the surface of said mesh elec- Y trodemember facing said electron guns and exhibiting field induced andcontrolled conduc- Y tivity;

(E) means including said storage target for forming a pattern ofelectrical charges on the surface of said dielectric layer in responseto the impingement thereon of said scanning electron beam;

(F) means for selectively controlling the leakage of said electricalcharges to said electrode member comprising means for establishing apredetermined electrical iield across said dielectric layer.

4. A multi-mode direct-viewing half-tone storage display tubecomprising, in combination:

( A) a first electron gun for providing a scanning electron beam ofelemental cross-sectional area;

(B) a second electron gun for providing a flood beam of electrons;

(C) a viewing target positioned within the target area of said electronguns adapted to luminesce in response to the impingement of electronsthereon;

(D) a storage target disposed between said electron guns and saidviewing target and including:

(l) a mesh electrode member;

(2) a storage dielectric layer disposed on the surface of said meshelectrode member facing said electron guns and exhibiting both secondaryelectron emission and bombardment induced conductivity at diiferentenergy levels of said scanning electron beam, said storage dielectriclayer further exhibiting field induced and controlled conductivity;

(E) means including said storage target for forming a pattern ofelectrical charges on the surface of said dielectric layer in responseto the impingement thereon of said scanning electron beam;

(F) means for selectively controlling the leakage of said electricalcharges to said electrode member comprising means for establishing apredetermined electrical field 4across said dielectric layer.

5. A direct-viewing half-tone storage display tube comprising, incombination:

(A) a rst electron gun for providing a scanning electron beam ofelemental cross-sectional area;

(B) a second electron gun for providing a ood beam of electrons;

(C) a viewing target positioned within the target area of said electronguns adapted to luminesce in response to the impingement of electronsthereon;

(D) and a storage target disposed between said electron guns and saidviewing target and including:

( 1) an electron permeable electrode member;

(2) a storage dielectric layer on the surface of said electrode memberfacing saidelectron guns 7 and exhibiting eld induced and controlledconductivity;

(E) means including said storage target for forming a pattern ofelectrical charges on the surface of said dielectric layer in responseto the impingement thereon of said scanning electron beam;

(F) means for selectively controlling the leakage of said electricalcharges to said electrode member comprising means for impressing apredetermined potential on said electrode member.

8 References Cited UNITED STATES PATENTS 3,089,055 5/1963 Lehrer 315-12JOHN W, CALDWELL, Acting Primary Examiner.

DAVID G. REDINBAUGH, Examiner.

T. A. GALLAGHER, R. K. ECKERT, IR.,

Assistant Examiners.

1. A DIRECT-VIEWING HALF-TONE STORAGE DISPLAY TUBE COMPRISING, INCOMBINATION: (A) A FIRST ELECTRON GUN FOR PROVIDING A SCANNING ELECTRONBEAM OF ELEMENTAL CROSS-SECTIONAL AREA; (B) A SECOND ELECTRON GUN FORPROVIDING A FLOOD BEAM OF ELECTRONS; (C) A VIEWING TARGET POSITIONEDWITHIN THE TARGET AREA OF SAID ELECTRON GUNS ADAPTED TO LUMINESCE INRESPONSE TO THE IMPINGEMENT OF ELECTRONS THEREON; (D) A STORAGE TARGETDISPOSED BETWEEN SAID ELECTRON GUNS AND SAID VIEWING TARGET ANDINCLUDING: (1) AN ELECTRON PERMEABLE ELECTRODE MEMBER; (2) A STORAGEDIELECTRIC LAYER ON THE SURFACE OF SAID ELECTRODE MEMBER FACING SAIDELECTRON GUNS AND EXHIBITING FIELD INDUCED AND CONTROLLED CONDUCTIVITY;(E) MEANS INCLUDING SAID STORAGE TARGET FOR FORMING A PATTERN OFELECTRICAL CHARGES ON THE SURFACE OF SAID DIELECTRIC LAYER IN RESPONSETO THE IMPINGEMENT THEREON OF SAID SCANNING ELECTRON BEAM; (F) MEANS FORSELECTIVELY CONTROLLING THE LEAKAGE OF SAID ELECTRICAL CHARGES TO SAIDELECTRODE MEMBER COMPRISING MEANS FOR ESTABLISHING A PREDETERMINEDELECTRICAL FIELD ACROSS SAID DIELECTRIC LAYER.